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The last time was during the election of 1824 when none of the four candidates achieved a majority and the House of Representatives met in January of 1825 to resolve the outcome. Although Andrew Jackson had won the most electoral votes, the House of Representatives awarded the presidency to John Quincy Adams.

If neither Democrat Hillary Clinton nor Republican Donald Trump reach the required 270 votes, the job of deciding the election goes to Congress. The rules of the process are dictated by the 12th Amendment.

"If no candidate receives a majority of electoral votes, the House of Representatives elects the president from the three presidential candidates who received the most electoral votes," the National Archives explains. "Each state delegation has one vote."

Based on the odds presented by FiveThirtyEight, this scenario would have the House choosing between Trump, Clinton and a third party candidate.

But that might not be the end of the turmoil. With 50 states each getting one vote, it would again be possible to have a 25-25 tie.

"If the House of Representatives fails to elect a president by Inauguration Day, the vice-president elect serves as acting president until the deadlock is resolved in the House," the National Archives explains.

A slightly smaller pool of candidates would be available for the role of vice president.

"The Senate would elect the Vice President from the 2 Vice Presidential candidates with the most electoral votes. Each senator would cast one vote for Vice President," according to the National Archives.

Additionally, FiveThirtyEight predicts an 8.9 percent chance of the popular vote in a key state being so close that a recount is required. Such a recount could last until late November or even into December, depending in part on the litigation which would likely reflect what the nation saw in 2000.

Additionally, a very close result or a tie within a state might force a state to hold a runoff election, the National Archives said. Those procedures would be governed by the laws of the individual states, but the procedure must be complete six days before the Electoral College meets.

Would a recount or run-off help to resolve the Electoral College results? In any of these scenarios, only this is certain: After a long campaign, we should all be prepared for the possibility that it isn’t over yet.